This invention relates to electronic digital communication systems and more particularly to radiotelephone systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards. This application focusses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users in the downlink (DL) direction and the users implement the commands in the uplink (UL) direction and vice versa. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a WCDMA communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs mobile station (MS), or user equipment (UE), calls via the appropriate base station(s) (BSs), which communicate with each UE through DL, or forward, and UL (i.e., mobile-to-base, or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS, which is called a Node B in 3GPP parlance, serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12,14 are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
High-speed downlink packet access (HSDPA) is a further evolution of WCDMA communication systems that provides higher bit rates, e.g., up to more than 10 megabits per second (Mb/s), by using higher order modulation, e.g., 16-ary quadrature amplitude modulation (16-QAM), multiple spreading codes, e.g., up to fifteen codes with spreading factors of 16, and DL-channel feedback information. HSDPA is described in the Release 5 version of the system specifications promulgated by the 3GPP. The DL-channel feedback information is information sent by a UE to a BS through the UL channel regarding the DL channel's quality. The BS uses that information to optimize the DL modulation and coding for optimized throughput.
HSDPA also employs a hybrid automatic repeat request (ARQ) scheme on the physical layer in order to reduce the round-trip delay of erroneous received packets. The hybrid ARQ scheme involves transmission by the UE of acknowledgment (ACK) and non-acknowledgment (NACK) messages to the BS providing HSDPA service. This BS may be called the “serving” BS or cell. The HS-channels in the DL are transmitted only from the HSDPA serving cell, and HSDPA UL control signaling (including ACK/NACK and DL-channel quality reports) is detected by only the HSDPA serving cell.
As user terminals move with respect to the base stations, and possibly vice versa, on-going connections are maintained through a process of hand-off, or handover. For example in a cellular telephone system, as a user moves from one cell to another, the user's connection is handed over from one base station to another. Early cellular systems used hard handovers (HHOs), in which a first cell's base station (covering the cell that the user was leaving) would stop communicating with the user just as the second base station (covering the cell that the user was entering) started communication. Modern cellular systems typically use diversity, or soft, handovers (SHOs), in which a user is connected simultaneously to two or more base stations. In FIG. 1, MSs 28, 30 are shown communicating with plural base stations in diversity handover situations. MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control communication link between the RNCs 12, 14 permits diversity communications to/from the MS 30 via the BSs 20, 22.
During SHOs, terminals receive TPC commands from more than one base station, and methods have been developed for handling conflicts between TPC commands from different base stations. Conflicts are expected because as a UE leaves one cell, that cell's base station receives a progressively weaker signal and thus that base station's TPC commands call for more power, and at the same time, the UE may be entering a new cell, and the new cell's base station receives a progressively stronger signal and thus the new base station's TPC commands call for less power. In a 3GPP-compliant system, the UE combines TPC commands from reliable downlinks with a logical OR function, which leads to reduced UE transmit power if any of the reliable commands says “DOWN”. This is described in Section 5.1.2.2.2.3 of 3GPP Technical Specification (TS) 25.214 (V6.2.0) Rel. 6 (2004), Physical layer procedures (FDD).
HSDPA can be used in mobility situations, e.g., where a UE and the BS(s) move with respect to one another, but soft handover is not specified for HSDPA channels. HSDPA channels support only hard handover. Therefore, there can be many situations in which a UE uses SHO for its DPCH(s) at the same time that it uses HHO for its HSDPA channel(s). FIG. 2 depicts a typical one of those situations in which a UE is in a SHO situation for non-HSDPA channel(s) and is using services transported through HSDPA channels.
FIG. 3A is similar to FIG. 2 in that it depicts a UE 202 having multiple simultaneous connections with BS 204 and a BS 206 via dedicated physical data channels (DPDCHs) and dedicated physical control channels (DPCCHs) in the UL and the DL. In other words, the UE 202 is in SHO with respect to these non-HSDPA channels. The DPDCH carries higher-layer network signaling and possibly also speech and/or video services. The DPCCH carries physical-layer control signaling (e.g., pilot symbols/signals, TPC commands, etc.). An RNC 208 (not shown in FIG. 3A) controls BS 204 and BS 206.
The UE 202 also has HSDPA channels, but these are provided by only the serving cell, which in FIG. 3A is BS 206 because the SIR of BS 206 is larger than the SIR of BS 204. As noted above, SHO is not specified for the HSDPA channels. The downlink HSDPA channels include an HS-Packed Data Shared Channel (HS-PDSCH) that carries HS data packets and the HS-Shared Control Channel (HS-SCCH) that carries control information for the data packets. The uplink HSDPA channels include an HS-Dedicated Physical Control Channel (HS-DPCCH) that carries the ACK/NACK reports and DL-channel quality information.
Although SHO is not available for HSDPA channels, the UE measures the average SIR (e.g., EC/I0) of the Common Pilot Channels (CPICHs) it receives from all cells in its Active Set on a regular basis (typically five times per second), and the cell having the best SIR on these non-HSDPA channels is designated as the HSDPA serving cell.
As depicted in FIG. 3A, the UE 202 determines an average SIR of the DL from BS 204 that is larger than the SIR measured for BS 206. This triggers an event 1D (change of best cell) and transmission of a Layer-3 radio resource control (RRC) message on the UL DPDCH. For a short time after the event 1D is triggered, the HS channels are still transmitted from the BS 206. The RNC receives the event-1D message and transmits a “change of HS serving cell message” to the UE as a Layer-3 RRC message on the DL DPDCH. The “change” message includes information about a time instant at which the HS channels will (hard) hand over to the BS 204. When the UE has received the “change” message, it transmits an ACK message on the UL DPDCH to the BSs 204, 206 and the RNC 208. In FIG. 3B, the HSDPA HHO has taken place, and the BS 204 is the serving cell transmitting and receiving the HS channels.
The UE's measurements of average SIRs of DL non-HSDPA channels can cause anomalies in HSDPA operations. It can sometimes briefly be so that the BS 204 has a better SIR than the BS 206. In addition, the UL and DL fade independently of each other, and therefore it can also be so that the UL to BS 204 has better quality than the UL to BS 206 even while the DL from BS 204 has lower quality than the DL from BS 206.
As mentioned above, the DPDCHs/DPCCHs are under transmit-power control and support SHO, and so the power control during SHO is based on a combination of TPC commands. The HS-DPCCH is power-controlled, but with an offset to the DPCCH UL that is set by higher layer signaling. While the DPCHs are in soft handover and considering the independent fading of channels, the combination of the TPC commands may be driven by base stations that do not include the HSDPA serving cell and so the HSDPA power control may be inappropriate. Indeed, in order to have the SHO capacity gain, it is sufficient if only one BS can hear the UE sufficiently well to achieve sufficient quality of service, and thus it could be so that the UL to the HSDPA serving cell is the cell having the weakest signal and another UL to a non-serving cell is the UL that directs power control on the HSDPA channel(s).
These behaviors can result in poor HS-DPCCH reception performance and erroneous ACK/NACK messaging and DL-channel quality detection, all of which can significantly reduce throughput on the HSDPA channel(s). Accordingly, attempts have been made to eliminate these problems.
One approach, taken by the 3GPP standards, is to specify a transmit power on the HS-DPCCH that is greater than the transmit power on the DPCHs. Nevertheless, all UL synchronization (i.e., path searcher and channel estimation) is done on the UL DPCCH. Therefore, if the reception of the DPCCH becomes poor enough, a loss of synchronization can result and the HS detection cannot be done regardless of the HS-DPCCH power! Lower HS throughput is a result.
Another approach is to do UL power control only on the HS serving cell, but doing so loses the SHO gain and greatly reduces the system's communication capacity. Therefore, this approach is not allowed by the 3GPP standards.
Another approach is to vary the data rate on the HS channels according to the channel quality. European Patent Application No. EP 1363413A1 by Hayashi et al., for example, describes a mobile communication system that uses the required transmission power on a DPCH as a control indicator for varying the data rate of the HS-PDSCH. As stated in the document, the HS radio link condition is expected to be good with the DL transmission power of a DPCH at a low level, and therefore fast transmission of data signals can be implemented and the risk of degradation of communication quality is low even if the transmission rate of HS-PDSCH is set high. Conversely, the radio link condition is expected to be bad with the DL transmission power of the DPCH at a high level, and therefore adequate communication quality cannot be maintained unless the transmission rate of data signals with HS-PDSCH is lowered.
Aspects of power control of HS channels during SHO of DPCHs are described in a number of documents, including U.S. Patent Application Publication No. US 2004/0203985 by Malladi et al. and International Patent Publication No. WO 2004/019513A1 to Whinnet et al. The document by Malladi et al. states that uplink power control is provided to maintain the integrity of the uplink HS-DPCCH when a UE goes into SHO. A RNC controls a target signal-to-noise-ratio threshold of a pilot signal based on the pilot signal strength of the serving node and/or the uplink channel condition of the serving node.